New topologies for planar microwave band-pass filters with strongly asymmetric characteristics

In this paper the problem of designing simple low-cost planar microwave band-pass filters with strongly asymmetric characteristics, filters that can very efficiently attenuate an adjacent channel, is investigated. For a fourth-order filter, the usual quadruplet structure leads to two transmission zeros, located one on each side of the passband. For one of the two adjacent channels, a higher attenuation can be obtained by designing filters with two transmission zeros with prescribed positions in the same stopband (upper or lower). The new topology investigated in this paper offers the possibility of designing such quadruplets with two transmission zeroes situated both on the same side of the passband, in prescribed and controlled positions. Filters with this novel structure can provide very high attenuation not only at a single frequency, but in a larger adjacent frequency band.